1. Technical Field
The present invention relates to a projector and an anamorphic prism optical unit, and, more particularly to a projector including an anamorphic prism optical system for converting an aspect ratio of an irradiation region.
2. Related Art
A projector modulates light using a spatial light modulating device having fixed size. On the other hand, in some case, input signals having various aspect ratios (vertical to horizontal ratios) are input to the projector. As measures against a problem in that aspect ratios are different between the spatial light modulating device and an image signal, in general, luminance in an image blank region (a blank section) caused by the difference between the aspect ratios is set to a minimum value. In other words, “black” is always displayed in the image blank region. However, if an image is displayed only in a section other than the blank section, a drop in resolution and a drop in light use efficiency are caused. Therefore, it is desired to solve the problem with other measures.
As a method for solving the problem, there is known a method of, after expanding the image to the image blank region of the spatial light modulating device and modulating the light, restoring the image to the original aspect ratio defined by the image signal. For conversion of an aspect ratio, for example, an anamorphic lens of a front converter type attached in front of a projection lens (on a surface to be irradiated side) is used. In this case, the conversion of an aspect ratio is performed according to only a peculiar magnification set in the front converter. Therefore, for example, when image signals of various standards (Cinema Scope, European Vista, American Vista, Standard, etc.) having relatively small aspect ratio differences are input as in movies, it is difficult to accurately apply the respective standards. To deal with this problem, for example, U.S. Pat. No. 4,017,160 proposes a technique for respectively rotating and displacing two prism sets provided in an optical path to thereby make it possible to continuously change and set an aspect ratio.
In the technique proposed in U.S. Pat. No. 4,017,160, a configuration in which two kinds of glass materials are bonded to each other is adopted as each of the prism sets, whereby a reduction in color aberration is realized. However, whereas such a configuration can satisfy conditions for color aberration reduction when rotation amounts of prisms are in a specific relation with each other, in some case the conditions for the color aberration reduction are not satisfied when the relation is changed according to the rotation of the prism sets.